1. Field of the Invention
The present invention relates to a lithium secondary battery and a method of manufacturing the lithium secondary battery, and more particularly to a lithium secondary battery which prevents the increases in electrode impedance resulting from expansion and contraction of electrode active material due to repeated charging and discharging, and to a method of manufacturing this lithium secondary battery. The present invention also relates to a high energy-density lithium secondary battery having increased sites in which lithium ion can be intercalated and de-intercalated, the increased sites increasing the capacity of positive electrode and negative electrode.
2. Related Background Art
Recently, it has been said that increasing CO2 gas contained in air exhibits green house effect to cause global warming. Thermal power plants use fossil fuel to convert thermal energy into electric energy. Accordingly, a large amount of CO2 gas is exhausted, being a barrier in building additional thermal power plants. Therefore, so-called load leveling has been proposed for effective use of electric power generated in the thermal power plants. That is, electric power generated in the night is stored in storage batteries at consumer""s homes and the stored electric power is used in the daytime during when electric power consumption increases, thereby leveling load distribution.
For electric vehicles having a feature where substances containing COx, NOx, and CH that contaminate air are not exhausted, the development of a high energy-density secondary battery has been demanded. In addition, the development of small size, lighter weight, high performance secondary batteries is urgently demanded which find applications in portable equipment such as book size personal computers, word processors, video cameras, and mobile telephones.
After JOURNAL OF THE ELECTROCHEMICAL SOCIETY 117, 222 (1970) has reported an application of graphite intercalation compound for a negative electrode of a lighter weight, smaller size secondary battery, a rocking chair type secondary battery referred to as a lithium ion battery has been developed and some have been put in practical use. This type of secondary battery uses a carbon material for a negative active material, and an intercalation compound containing lithium ion for positive active material. With this lithium ion battery, the negative electrode is formed of a host material in the form of a carbon material that allows lithium ions as a guest material to be intercalated. The use of such a material suppresses dendrite growth of lithium during the charging of the battery, thereby allowing more number of charging/discharging cycles in the useable life of the battery.
Since the aforementioned lithium ion battery achieves a long-life secondary battery, proposal and research are carried out vigorously in an attempt to apply various carbon materials to the negative electrode. Japanese Patent Application Laid-Open No. 62-122066 proposes a secondary battery using a carbon material where an atomic ratio hydrogen/carbon is less than 0.15, the distance between (002) planes is 0.337 nm or longer, and the crystallite size in c-axis is 15 nm or less. Japanese Patent Application Laid-Open No. 63-217295 proposes a secondary battery using a carbon material where the distance between (002) planes is 0.370 nm or longer, true density is less than 1.70 g/ml, and a peak value of heat generated is 700xc2x0 C. or higher when subjected to differential thermal analysis in flowing air. There are some research reports on the application of various carbon materials to negative electrode. Carbon fibers are reported in Electrochemical Society Vol. 57, p.614 (1989). Natural graphite is reported in the Proceedings of the 33rd Battery Symposium, Mesofuse microsphere and graphite whisker are reported in the Proceedings of the 34th Battery Symposium, p.77 (1993) and p.77, respectively. Burned furfuryl alcohol resin is reported in the Proceedings of the 58th Conference of the Electrochemical Society of Japan p.158 (1991).
However, with a lithium ion battery which uses a carbon material containing lithium as a negative electrode active material therein, there has been developed no battery whose discharge capacity exceeds the theoretical value of the graphite intercalation compound, the discharge capacity being such that a stable electric power can be drawn from the battery when the battery is used through repeated charging and discharging. That is, the theoretical value is such that a carbon intercalation compound can store one lithium atom for every six carbon atoms. Thus, a lithium ion battery using a carbon material as a negative active material has a long cycle-life but not as large an energy density as a lithium battery that directly uses metal lithium as a negative active material. If the negative electrode of a lithium ion battery formed of a carbon material is to be intercalated with lithium of an amount larger than the theoretical capacity during charging cycle, lithium metal grows in a dendrite pattern on the surface of the negative electrode formed of a carbon material, ultimately causing an internal shorting out between the negative electrode and positive electrode due to repeated charging and discharging cycles. A lithium ion battery with the theoretical capacity of a graphite negative electrode has not a long enough cycle life for practical use.
On the other hand, a high capacity lithium secondary battery that uses metal lithium for negative electrode has been demanded but not put in practical use yet. Because the charging/discharging cycle life is very short. This short cycle life is considered to be primarily due to the fact that metal lithium reacts with impurities such as moisture contained in the electrolyte to form an insulating film on the electrodes and therefore repeated charging and discharging causes lithium to grow in a dendrite pattern, resulting in an internal shorting out between the negative and positive. This leads to the end of the battery life.
If a dendrite pattern of lithium grows to short-circuit negative electrode and positive electrode, the energy stored in the battery is consumed in a short time so that heat is generated and the solvents of the electrolyte are decomposed to generate gas to increase internal pressure, thereby damaging the battery.
In order to alleviate the problem of metal lithium negative electrode that a metal lithium reacts with the moisture and organic solvents contained in the electrolyte, use of a lithium alloy containing lithium and aluminum also has been proposed. However, use of a lithium alloy is not currently in practical use due to the following problems. A lithium alloy is too hard to be wound in a spiral form, and therefore a spiral cylindrical battery cannot be made. The charging/discharging cycle life is not prolonged as much as one expects. A battery using a lithium alloy for negative electrode does not provide as much energy density as a battery using metal lithium.
Japanese Patent Application Laid-Open Nos. 5-190171, 5-47381, 63-114057, and 63-13264 have proposed the use of various forms of lithium for negative electrode. Japanese Patent Application Laid-Open No. 5-234585 proposes the application of metal power on the surface of lithium, the metal powder preventing lithium from producing various kinds of intermetallic compounds. None of the proposals in the aforementioned publications can be a decisive answer that prominently prolongs the life of the negative electrode.
JOURNAL OF APPLIED ELECTROCHEMISTRY 22 (1992) 620 to 627 reports a high energy density lithium secondary battery using an aluminum foil for negative electrode, the lithium secondary battery having an energy density lower than a lithium primary battery. When this lithium secondary battery is subjected to as many charging/discharging cycles as practical, the aluminum foil experiences expansion and contraction repeatedly till the aluminum foil is finally cracked, leading to reduced current collection and dendrite growth. Thus, a secondary battery having a practically long life has not been developed yet.
For these reasons, there is a strong demand on the development of material for the negative electrode which has a longer life and a higher energy density than the negative electrode of carbon currently in practical use.
In order to implement a high energy-density lithium secondary battery, the development of materials for not only negative electrode but for positive electrode are necessary. At present, a lithium-transition metal oxide is most commonly used as an active material for a positive electrode, the lithium-transition metal oxide having lithium ion inserted (intercalated) in an intercalation compound. However, the lithium-transition metal oxide can achieve a discharge capacity of only about 40 to 60% of the theoretical capacity. In particular, in order for a battery to be a practical battery having a long charging/discharging cycle life, the charging/discharging capacity should be as low as possible. This is detrimental to the implement of high capacity battery. For example, the 34th Battery Symposium 2A04 (pp.39-40) reports that when the cobalt acid lithium is charged so that lithium is de-intercalated more than xc2xe of the theoretical capacity, the crystal structure of the cobalt acid lithium changes from single crystal to hexagonal system. The c-axis extremely shrinks during the intercalation with the result that the reversibility of lithium becomes extremely deteriorated from the next discharge onward. Thus, charging/discharging cycle property deteriorates. This is true of, for example, nickel acid lithium.
In order to suppress changes in crystal structure, for example, the 34th Battery Symposium 2A08 (pp.47-48) proposes that a portion of lithium contained in cobalt acid lithium is substituted by sodium, potassium, copper, and silver. Adding cobalt, manganese, aluminum or the like to nickel acid lithium also has been reported. However, these proposals are not enough for improving the utilizing efficiency and charging/discharging cycle characteristic.
As mentioned above, with a lithium secondary battery, including a lithium ion battery, which uses lithium ion as a guest material for charging/discharging reaction, there have been strong demands for the development of negative and positive polarities having a practical life, the negative and positive polarities having a higher capacity than a negative electrode of a carbon material and a positive electrode of a transition metal oxide which have currently used.
The present invention was made in view of the aforementioned problems.
An object of the invention is to provide a method of manufacturing a lithium secondary battery which uses oxidization and reduction of lithium ion, the secondary battery having a positive electrode formed of a high capacity positive electrode active material and a negative electrode formed of a high capacity negative electrode active material.
Another object of the invention is to provide a lithium secondary battery including at least negative electrode, positive electrode and electrolyte, and using the oxidization and reduction of lithium ion, the negative and/or positive polarities being formed of an active material having at least an amorphous phase. (a) The composition of the active material is a material which has at least an amorphous phase and contains at least one of cobalt, nickel, manganese, and iron that have an amorphous phase. The active material having a half value width not less than 0.48 degrees, the half value width being a diffraction angle that half a peak value of the highest of diffraction intensity appearing on an X-ray diffraction chart. The diffraction intensity is plotted against X-ray diffraction angle (2xcex8).
Another object of the invention is to provide a lithium secondary battery including at least negative electrode, positive electrode, and electrolyte, and using the oxidization and reduction of lithium ion, wherein (b) the negative electrode is made of an active material having at least an amorphous phase and a half value width not less than 0.48 degrees, the half value width being a diffraction angle that half a peak value of the highest of diffraction intensity occupies. The diffraction intensity appears on an X-ray diffraction chart in which diffraction intensity is plotted against X-ray diffraction angle (2xcex8). The active material is a composite material of an amorphous material and a second material, the amorphous material having an amorphous portion and the second material containing at least one of carbon and metal elements which have an amorphous phase and are electrochemically inert to substances other than lithium during the charging/discharging reaction of the lithium battery.
The present invention provides a method of manufacturing a lithium secondary battery characterized in that an amorphous material is prepared by giving physical energy to a crystalline material, and the amorphous material is used as a positive active material to form a positive electrode and/or as a negative active material to form a negative electrode.
In the present invention, the term xe2x80x9cactive materialxe2x80x9d is used to cover substances that contribute to the electrochemical reaction (repeated reaction) of charging and discharging a battery.
The present invention provides a lithium secondary battery which includes at least a negative electrode, a positive electrode, and an electrolyte, and uses the oxidization and reduction of lithium ion. Electrodes are formed of active materials having at least an amorphous phase, wherein the active material is a compound having an amorphous phase and containing at least one or more elements selected from cobalt, nickel, manganese, and iron. The active material has a half value width not less than 0.48 degrees, the half value width being a diffraction angle that half a peak value of the highest of diffraction intensity occupies. The diffraction intensity appears on an X-ray diffraction chart in which diffraction intensity is plotted against X-ray diffraction angle (2xcex8).